Sega of America today revealed plans for an "Explosive Announcement Event" on May 12th, the first day of E3. The announcement describes a "mystery briefing" that will contain "explosive news -- something even YOU won't be able to guess!"
This hasn't stopped the masses from speculating though. Due to the use of "explosive", many are theorizing that this mystery announcement has something to do with an upcoming title featuring, um, explosions. Others postulate that Sega will be announcing a new console, but if that's the case why would they be showing multi-console titles at the show?
There aren't any definite details at this point, so we'll likely have to wait until the 12th to see exactly what Sega has planned.